Satellite geopositioning (or geolocation) devices, referred to by the more complete name of satellite positioning and dating system or by the abbreviation GNSS for Global Navigation Satellite System, compute the position of the reception terminal by measuring the propagation time of the signals transmitted by the positioning satellites between the satellites and the reception terminal. Each geopositioning satellite transmits a coded message containing a certain number of information items, including its own position and the exact instant of transmission of the signal. In order to estimate its position, the geopositioning terminal measures the time that has elapsed between transmission and reception of the coded message and thereby deduces therefrom the distance that separates it from the satellite. Measurements on at least four satellites provide the distances that are necessary for determining the three coordinates of the position, namely latitude, longitude and altitude. This type of measurement is known by the designation “pseudo-distances” in GPS terminology.
In what is known as a constrained environment, that is to say one in which reception conditions for the GNSS signals are not favourable, for example because of multipath problems, masking or interference, the position estimated by the terminal is not always exact and the user has no information about the quality of the computed position.
At present, there is a GNSS augmentation system, or SBAS for Satellite Based Augmentation System, which allows the precision of the GPS (Global Positioning System) to be improved by reducing the margin of error of the system. This augmentation system watches over the GPS system and sends the user information about the quality of the positioning satellites, about the quality of the propagation of the signals and correction values. The aim of these algorithms is to provide the user with a PVT+I (Position, Velocity, Time, Integrity) solution in which the integrity arises from the use of estimation of a quality indicator about the state of the GNSS system, typically about the orbital synchronization of the satellites, but also estimation of the propagation error caused by the ionosphere. Thus, instead of computing his position, the user computes the bubble that he is situated in, that is to say his position plus an estimate of the error that has been made.
A disadvantage of this system is that it has been designed for civil aviation and does not work correctly for a user on the ground. The reason is that on the ground, and notably in an urban environment, there are local propagation phenomena that a system for watching over the GPS constellation cannot anticipate and therefore cannot correct. Among the local propagation phenomena in an urban environment, it is possible to cite masking or multipath phenomena linked to the presence of buildings, for example.
In the prior art, there is equally a RAIM (Receiver Autonomous Integrity Monitoring) algorithm, which is capable of detecting the failure of a geopositioning satellite and of excluding this broken down satellite so that the GPS receiver no longer takes account of the erroneous data that it transmits. The technique involves mixing all of the geopositioning signals that are available for computing a position. By virtue of the measurement redundancy, the algorithm then evaluates the consistency between the various pseudo-distances computed on the basis of the estimated point. The main limitation of this technique is the fact that it requires a large number of measurements. It is therefore not suited to being used in the presence of a high level of masking.
Technologies of “batch” type are likewise known in the prior art, notably from a publication by Mezentsev. These technologies have been developed with the aim of loosely crossing an inertial sensor and a number of GNSS measurements that is too small to compute an instantaneous position. The aim of the technology is therefore to propagate the difference in the position of the user between two instants via the inertial sensor of the geopositioning terminal. This propagation is then used to establish the position, speed and the exact time (PVT for Position, Velocity and precise Time) over all of the pseudo-distances accumulated in the course of time.
A problem arises when the geopositioning terminal 10 is not equipped with an inertial unit.